1. Field of Invention
The present invention relates to a toner for electrophotography also capable of being applied to an electrostatic recording method, a developer for electrophotography employed to develop an electrostatic latent image, and a method for forming an image employing the same.
2. Description of Related Art
In electrophotography, an electrostatic latent image formed on a photoconductor is developed using a toner containing a binder resin and a colorant, and a toner image thus obtained is transferred onto a transfer paper and fixed using a heating roll to obtain an image. A dry type developer employed in electrophotography can broadly be grouped into a one-component developer containing only toner, which is a colorant dispersed in a binder resin, or a two-component developer containing such toner together admixed with a carrier.
In late 1980's, a more compact device having higher performance was in demand in the electrophotography market, especially with regard to full color image quality where the quality equivalent to that of art printing or silver chloride photography is desired. Digital conversion is essential to achieve such high image quality, and it allows complicated image processing to be done rapidly. Digital conversion enables control of a character that is independent of a graphic image, and provides substantial improvement in image reproducibility when compared with analog technology. Especially with regard to a photographic image, it is very valuable that gradation correction and color correction are possible, and gradation characteristics, minute line reproducibility, sharpness, color reproducibility and graininess are also advantageous when compared with an analog image.
Recently, electrophotography began to also be employed in office printing, especially in DTP (desktop publishing) as a result of the advanced technology described above. In such case, on demand printing system is an advantageous property. However, such systems require a developer to give a correct output of a latent image originally formed in an optical system, which leads to the employment of a toner having a smaller particle size, which then gives rise to increased significance of improved basic characteristics in development, transfer and fixing.
A thermoplastic resin has conventionally been employed as a toner for electrophotography. For the purpose of achieving both low energy fixing and proper particulate blocking characteristics, the rheology and the glass transition point (hereinafter abbreviated as Tg) of a resin used as a toner is optimized as disclosed in Japanese Patent Application Publication No. 2-37586 and Japanese Patent Application Laid-Open No. 1-225967 and No. 2-235069. Among such optimized resins, one having a lower Tg is usually employed for toner for the office printing system described above because rapid fixing can be achieved.
However, an image formed using a toner obtained by the method described above undergoes melting of the resin component in the image region when subjected to heat at a temperature close to Tg or higher, resulting in adhesion of the resin onto the reverse side of a copy or onto another printing material, whereby causing a blank area in the image. In addition, in office printing, where two-sided printing is frequent, the image regions are in contact with each other, resulting in more frequent formation of blanks in the image when compared with one-sided printing.
For the purpose of office printing such as DTP as described above, the form of a printed material, which has frequently been subjected to two-sided printing and then bound into a book, makes the stability of the print quality over a prolonged storage period and during shipping very important.
Japanese Patent Application Laid-Open No. 4-186368, which is intended to avoid offset upon fixing, describes a method wherein a thermosetting resin is added externally to a toner and allowed to undergo a curing reaction with a polyester binder resin, whereby attempting to avoid offset upon fixing and attempting to improve low temperature fixing and OHP transparency. In this method, the polyester resin is melted upon fixing and the melted polyester undergoes crosslinking with a microparticle of an epoxy resin or a melamine resin to increase the internal cohesive force of a toner layer, whereby preventing offset upon fixing. By this method, it is actually possible to prevent offset as a result of increased internal cohesive force of the toner due to the partial crosslinking of the resin. However, in view of stability over a prolonged storage period and during shipping no sufficient print quality can be obtained. In addition, this method involves the problems of irregular image surface due to the partial increase in the pile height of the print image since the thermosetting resin microparticles undergo crosslinking between the thermosetting resin particles instead of crosslinking with a toner binder resin. Furthermore, this process also involves the problem of a blanked image due to the release of the crosslinked thermosetting resin microparticles from the image surface over a prolonged period of storage of the printed material, suggesting that it is not suitable to the office printing application.
Japanese Patent Application Laid-Open No. 2-101477 discloses a method in which the surface of a toner is cured with a polyurethane resin and an adhesiveness-imparting resin is contained internally together with a binder resin to achieve both anti-offset capability and low temperature fixing ability. In this method, the surface of an image becomes disadvantageously irregular due to curing of the toner surface. Accordingly no satisfactory print quality, in terms of stability over a prolonged storage period and during shipping, can be obtained.
The present invention is established in view of the current technical circumstance described above.